Diverter switch for on-load tapchanger with conductive rail support means



Aug. 24, 1965 A. J. HASELFOOT ETAL 3,202,776

DIVERTER SWITCH FOR ON-LOAD TAP-CHANGER WITH CONDUCTIVE RAIL SUPPORTMEANS 4 Sheets-Sheet 1 Filed July 51, 1963 Aug. 24, 1965 A. J. HASELFOOTETAL 3,202,776

- DIVERTER SWITCH FOR ON-LOAD TAP-CHANGER WITH CONDUCTIVE RAIL SUPPORTMEANS Filed July 31, 1963 4 Sheets-Sheet 2 g- 1955 A. J. HASELFOOT ETAL3,202,776

DIVERTER SWITCH FOR ON-LOAD TAP-CHANGER WITH CONDUCTIVE RAIL SUPPORTMEANS Filed July 51, 1963 4 Sheets-Sheet 3 United States Patent3,202,776 DIVERTER SWITCH FOR 0N -LOAD TAP- CHANGER WITH CONDUCTIVE RAILSUPPORT MEANS Arthur John Haselfoot, Keats Grove, London, and FrankRichard Scrutton, Cayhall, Ilford, England, assignors to Fuller ElectricLimited, London, England, a company of Great Britain Filed July 31,1963, Ser. No. 298,825 Claims priority, application Great Britain, Aug.1, 1%2, 29,522/ 62 11 Claims. '(Cl. 200-18) This invention relates to animproved electric load transfer switch and in particular, but notexclusively, to an improved diverter switch for an on-load tap-changer.

According to the invention an electric load transfer switch comprises acontact movable between first and second positions, means forcontinuously maintaining electrical contact with the movable contactthroughout its movement, a first stationary contact engageable by saidmovable contact in said first position and for a substantial part of themovement of the cont-act adjacent to said first position, a secondstationary contact engag able by said movable contact in said secondposition and for a substantial part of the movement of the contactadjacent to said second position, and means for ensuring that electricalcontact is maintained between said movable contact and each of saidstationary contacts during respective parts of the movement of themovable contact, at least one of the stationary contacts having, for atleast a part of that portion thereof which is engaged by the movablecontact, a resistivity which varies in the direction of movement of themovable contact.

The movable contact may be slidably mounted for reciprocating movementbetween .the first and second limiting positions. Alternatively, themovable contact may be rotatably mounted, the first and second positionsrepresenting points on the arcuate path traced by a portion of thecontact during its rotation. 1

The means for continuously maintaining electrical contact with themovable contact may, where possible, consist of a length of flexible,electrically conducting material attached to the movable contact.Alternatively, the means may comprise resiliently mounted electricalconducting brushes or rollers on the movable contact which brushes orrollers make continuous electrical contact with a conducting rail in allpositions of the movable contact.

The first and second stationary contacts may be disposed and dimensionedso that throughout a partof the movement of the movable contact betweenthe first and the second positions, the movable contact engages bothfirst and second stationary contacts.

The means for ensuring that electrical contact is maint-ained betweenthe stationary and movable contacts, during movement of the latter, maycomprise electrically conducting brushes or rollers on the movablecontact which, during movement of the movable contact, traverse asuitably prepared track on the respective stationary contact. Preferablythere would be a set of brushes or rollers for each stationary contact.

Preferably each stationary contact has for a substantial part thereof ofthat portion engaged by the movable contact, a resistivity which variesin the direction of movement of the movable contact.

According to a further feature of the invention a diverter switch for anon-load tap-changer comprises an electrically conducting carriagereciprocatively movable between first and second limiting positionswhere the carriage is positively engaged, respectively, by first andsecond pairs of contacts, means for maintaining electrical contactbetween the carriage and one contact of each pair of contacts in allpositions of the carriage intermediate the contacts, a first stationarycontact member electrically connected to the other contact of the firstpair of contacts and independently connected electrically to thecarriage when the latter is in the first limiting position andthroughout an initial part of movement of the carriage from the first tothe second limiting positions, a second stationary contact memberelectrically connected to the other contact of the second pair ofcontacts and independently connected electrically to the carriage whenthe latter is in the second limiting position and throughout the finalpart of the aforementioned movement, said first and second stationarycontact members each being constructed so that the resistivity of thatpart of each contact member which is electrically connected to thecarriage is difierent at different points along the respective contactmember in the direction of movement of the carriage, the sum of thelengths of the aforementioned initial and final parts of the movement ofthe carriage between limiting positions being greater than the totaldistance travelled by the carnage between limiting positions. Preferablyeach stationary contact member extends considerably more than half wayacross the gap between the pairs of contacts so that for a finitecentral portion of the movement of the carriage between the first andsecond pairs of contacts the carriage is making contact with both thefirst and second stationary contact members.

Conveniently the stationary contact members are bars extending int-o thespace between the pairs of contacts, the axes of the bars beingparallel. The part of the first contact member adjacent to the firstpair of contacts and the part of the second contact member adjacent tothe second pair of contacts are preferably of low resistivity material,the resistivity of each contact member increasing (in one example)towards the other pair of contacts.

In use the diverter switch would be associated with a tap selector whichlatter would connect one stationary contact member and the respectivecontact of the first pair of contacts to one transformer tapping and theother stationary contact member and the respective contact of the secondpair of contacts to an adjacent tapping.

In a conventional diverter switch the transition resistors which areconnected or disconnected across adjacent tappings or in series with theline current by the diverter switch are independent of the switch andare often mounted separately from it.

In a diverter switch in accordance with the invention the transitionresistors may be incorporated into the stationary contact members, themoving contact comprising conducting brushes or rollers sliding orrolling over the stationary contact members, each of which comprises arod or bar formed of special resistance material. By suitablydimensioning the rods or bars, initial movement of the carriage from thefirst contact to the second contact may cause the carriage to engage thesecond stationary contact member thereby inserting between adjacenttappings a resistance equal to the entire length of the rod or bar ofthe second stationary con-tact member. Throughout most of the remainderof the movement of the carriage the brushes or rollers slide or rollalong the rods or bars maintaining a determinable resistance, due inpart to each rod or bar, between the tappings until, close to the end ofthe movement of the carriage, full contact is made with the othertapping as the carriage is disconnected from the rod or bar of the firststationary contact member. One advantage of such an arrangement is thatthe current breaking duty imposed on the switch is considerably reducedif the resistance values of the rods or bars are suitably determined toeffect a continual and smooth transfer of current from one rod or bar tothe other.

The resistance between two points on a given rod or bar can be made tovary directly with the distance between the points, or according to someother law, e.g., as the square root of the distance. If the resistancevaries in a non-linear way in this manner it can be arranged that whencontact is first made and the resistance is connected bet-weenadjacenttappings, the total value of resistance can be quite high, thusreducing the current that has to be made and broken. The initialmovement of the sliding or rolling contacts can then be made to cut outhigh resistance values, and to insert small resistance values as far asthe mid-point'of the movement, and then to insert large resistancevalues and cut out small resistance values. By this means anotheradvantage is obtained, that the voltage drop in the external circuitduring the process of a tap-change can be considerably reduced.

The resistivity of each rod or bar can be made to vary continuously,thus following any predetermined theoretical curve, or it can be made tovary in steps, thus approximating to such a curve by segments ofstraight lines. The latter method is considerably easier in practice, asthe rod or bar can be composed of a number of small sections connectedin series, the resistivity of each section varying progressively alongthe length of the rod or bar.

The moving contacts on the carriage may be copper rollers mounted one ormore on each side of each fixed rod or bar. The rollers are convenientlyspring-loaded thus ensuring that there is no resultant lateral force onthe rod or bar. Conveniently, just before the instant of current breakwith either rod or bar, the current carried by the carriage to that rodor bar is tansferred from the copper'rollers to another roller which maybe mounted alongside the copper ones. This additional roller makescontact with a specially shaped portion of the rod or bar and allows thecopper rollers to leave the surface of the rod or bar. These currentbreaking parts, consisting of the additional roller and the speciallyshaped contact portion, are made of some arc-resistant material, e.g.,copper tungsten alloy or copper tungsten carbide alloy, and the currentbreaking parts are easily detachable for purposes of renewal. V

Spring-loaded brushes may bear on the copper rollers, the transfercurrent to or from the rollers. The contacts in which the carriage isengaged in each limiting position may be spring-loaded fixed clips of aheavy section, and ensure that the normal load current and any overloador short circuit current is not carried by the rollers.

The carriage of the diverter switch may be driven from one limitingposition to the other by any convenient means known per se. For examplethe carriage can be moved with snap-action between end limit positionsby an overcentre, stored energy mechanism.

One further advantage which may be derived from this form of diverterswitch is that the relative speed of movement of the current breakingparts at the moment of current break can be kept down to quite amoderate value, as the current to be broken can be limited to quitesmall values by the resistance of the stationary contacts in the mannerdescribed above.

One form of switch in accordance with the invention, intended as adiverter switch for an on-load tap-changer, will now be described, byway of example, with reference to the accompanying drawings, in which:

FIGURE 1 is a side elevation of the switch,

FIGURE 2 is a plan of the switch of FIGURE lrwith the contact clipsremoved,

FIGURE 3 is a sectional view of the switch .taken on the line III-4H ofFIGURE 1, and

FIGURE 4 is aschematic circuit diagram showing the mode of connection ofthe diverter switch of FIGURES 1 to 3 in a typical installation. 7

The switch, in essence, comprises a carriage (generally desi nated 1)slidably mounted on a base member 2 and adapted to be moved betweenfirst and second limiting positions.

The base member 2 is formed from a metallic base plate 3 supporting twospaced parallel upstanding electrically insulating plates 4 and .5. Alsoextending upwardly from the base plate 3 is a spaced pair of parallelmetal guideplates 6 and 7. The median plane of the insulating plates 4and 5 is normal to the median plane of the guide plates 6 and 7. Mountedon the base plate 3 and disposed to lie between the guide plates 6 and 7is a lower rack 8.

The carriage 1 is formed from a main electrically conducting supportplate 9 on the upper surface of which are mounted two spaced rollercradles 1t) and 11 and on the lower surface of which is mounted an upperrack 12 and spaced contact-carrying arms 13 and 14. The carriage 1 issupported on the base member 2 through the engagement of wings 15projecting outwardly from the upper rack 12 in keyways 16 and 17 in theguide plates 6 and 7. a

Rotatably mounted between the guide plates '6 and 7 to engage the upperand lower racks 8 and 12, respectively, is a spaced pair of pinions 18,the pinions 18 being mounted on a common spindle 19 extending at eachend into a slot 20 formed in each of the guide plates 6 and 7. Alsomounted on the spindle 2d and disposed between the pinions 18, is anoperating member 21.

From the description so far given, it will be appreciated that,referring to FIGURE 1, if the operating member Zll were to be moved agiven distance to the right, the effect would be to cause the pinions 18to roll, in a clockwise direction, over the fixed lower rack 8 and, inconsequence of the interengagement between the pinions 18 and the racks8 and 12, to cause the upper rack 12 to move to the right by an amountequal to twice the aforementioned given distance. Thus it follows thatmovement of the carriage between the plates 4 and 5 from a firstlimiting position (illustrated in the drawings) adjacent to plate 4, toa second limiting position adjacent to plate 5, can readily be effectedby moving the operating member 21 by a distance equal to half thedistance between the limiting positions of the carriage 1. Any suitablemeans (not shown) may be provided for driving the pinions 18, forexample, an over-centre, stored energy mechanism.

Mounted to extend between the plates 4 and 5 are spaced parallel contactmembers 22 and 23. Each contact member is formed from a copper end-block24, a plurality of electrically resistive blocks 25 secured inend-to-end abutting relationship and an electrically insulating endsupport 26. The assemblage of blocks 25 forming each contact member issecured together by means of an insulating rod (not shown) extendingaxially through the contact member, on which the various resistiveblocks 25 are slid, the rod being screwed into the end-block 24 at oneend and into the end support 26 at the other end. The resistivities ofthe individual blocks 25 forming each contact member are graded, so thatin moving along either contact member, the resistivity increases fromblock to block with the lowest resistivity block adjacent to the copperend-block 24. As can be seen from FIGURE 1, the thickness of the endsupportZd is less than the thickness of the blocks 24 and 25 andintermediate the insulating end support 26 and the adjacent resistiveblock 25, the contact member is formed from a central tapering plate 27and four small blocks 28 of arc-resistant material.

The contact members 22 and 23 are mounted between the plates 4 and 5 'sothat the copper end-block 24- of the member 22 is adjacent to the plate5 and the end-block 24 of the member 23 is adjacent to the plate 4-. Themode of attachment of the contact members to the plates 4 and 5 at thecopperblock end of the member, is made via conducting rods 2% whichfformelectrical terminals for making electrical connection to the respectivecontact member. V

The carriage l is electrically connected with each contact member 22 and23 by means of the aforementioned roller cradles and 11. Each rollercradle contains a plurality of electrically-conducting roller assemblies30, one half of the roller assemblies riding above the contact memberand one half of the roller assemblies located below the contact member.Each roller assembly 30 is formed by two or more central, copper,barrel-shaped rollers 31 and two outer discs 32 of an arc-resistantmaterial. The maximum radius of the central rollers 31 is slightlygreater than the radius of the discs 32. The roller assemblies aremounted on shafts 33 supported in elongated slots 33a in the cradles andare urged together in pairs against the upper and lower surfaces of thecontact member by means of tension springs 34. Good electrical contactbetween the individual roller assemblies 30 and the roller cradle 10 or11, is ensured by the provision of an electrically conducting-contactblade 35 for each roller assembly 30. Each blade 35 is bolted to therespective cradle and bears against the respective roller assembly.Bolted to the top of each cradle ltl and 11 is anelectrically-conducting contact blade 36.

Two pairs of electrically-conducting spring-urged clip contacts 37 aremounted on each plate 4 and 5, a first two pairs of clip contacts (i.e.,pairs 37a) being positioned and dimensioned to make good electricalcontact with the contact blades 36 when the carriage 1 is in one of itsend positions and the other two pairs (i.e., second pairs 37b)positioned and dimensioned to make contact with the blades 36 when thecarriage 1 is in the other end position. The clip contact 37b of thesecond pair which is mounted above the end-block 24 of contact member 22is electrically connected to that end-block by leads 41 and similarlythe clip contact 37a of the first pair which is directly above the otherend-block 24 is connected to that other end block by leads 42.

Mounted on the base plate 3 are two electrically-conducting pick-uprails 38 and 39, one rail being disposed on each side of the lower rack8. Mounted within each of the aforementioned contact-carrying arms 13and 14, is a pair of spring-urged sliding contacts 40. The rails aredisposed so that as the carriage 1 makes a stroke from one end positionto the other, the contacts 40 slide along the respective pick-up rail 33or 39 to maintain electrical contact between the carriage 1 and therails throughout the length of the stroke.

By making the barrel-shaped rollers 31 of slightly larger maximum radiusthan the discs 32 of arc-resistant material, and by providingarc-resistant blocks 28 on both sides of a central tapering plate 27 atthe high-resistive end of each contact member, it can readily bearranged that, as the carriage 1 moves between end positions, electricalcontact is made or broken with a contact member only via discs 32 andblocks 28. This can be better appreciated from a consideration of FIGURE2, Where itwill be seen that on contact member 23, the connectionbetween the last resistive block 25 and the insulating end support 26 isdimensioned so that as the carriage moves to the right and the rollersreach the end of the blocks 25, the central rollers 31 on the uppersurface of member 23 roll down the inclined surface of the taperedportion 2'7 and the central rollers 31 bearing against the underside ofthe member 23 move upwards on the inclined surface on the underside oftapered portion 27 until the discs 32 are resting on the blocks 23. Asmovement of the carriage 1 to the right continues, the discs 32eventually roll oif the blocks 2% altogether, thus breaking contactbetween the carriage and the contact member 23, whereupon the rollerassembly is supported slightly proud of the surface of the end support26 by virtue of the shafts 33 reaching the inner ends of the slots 33a.From an examination of the carriage 10 in FIGURE 3, it will be seen thatthe roller assemblies 30 are not resting upon the surface of the contactmember 22. Thus the make and break between the carriage 1 and either ofthe contact members 22 or 23 is always made via pieces of arc-re sistantmaterial. The gradation of resistivity as a function of distance alongthe blocks 25 of the contact members 22 and 23, is in fact discontinuousdue to each block being of substantially uniform resistivity and havinga finite width, but in practice however, resistivity along the contactmembers can be considered as varying approximately as the nth power ofthe distance along the memher, it lying between 1 and t).

In the particular switch illustrated, the blocks 25 are made of graphitebased materials. It will be appreciated, however, that they could beconstructed of suitable metallic alloy materials or alternatively couldbe made from resistance wire or strip wound into helical form andembedded or potted in a suitable electrical insulating composition(e.g., a hardened synthetic resin material). To obtain the requiredvariation of resistivity throughout the length of the contact member thewire or strip would either have to be fashioned with a varyingcross-sectional area from one end to the other, or else formed from anumber of separate lengths of wire or strip, each of a differingresistivity, joined together end to end. The arcresistant material weprefer to employ, is copper-tungsten alloy or copper-tungsten-carbidealloy.

One particular application of the diverter switch illustrated is foron-load tap changing and for this application the switch would beconnected with a suitable tap selector switch in the manner shown inFIGURE 4.

For convenience the tap selector has been assumed to be in the starpoint of a transformer winding with the diverter switch forming theneutral point of the high voltage supply (hereinafter called the H.V.neutral point), but the device could just as easily have been connectedin a different part of the winding. The two rails 38 and 39 areconnected together to the H.V neutral point and also to the two pairs ofclip contacts 37 which are not directly connected to the contact members22 and 23. One tapping (e.g., tapping A) of the transformer winding isconnected via the tap selector, to the copper end-block 24 of contactmember 23 and the pair of contact clips 37 connected thereto and theother tapping (tapping B) of the transformer winding is connected to thecopper end-block 24 of contact member 22 and to the contact clip pair 37associated therewith.

Let it be assumed that the carriage is in its illustrated position sothat through the carriage 1 and the contact clips 37a, the H.V. neutralpoint is solidly connected to tapping A, and that a switching actioncommences through actuation of the operating member 21.

The carriage 1 starts to move to the right so that the blades 36 arepulled from the jaws of the two pairs of clips 37a and electricalcontact between the carriage 1 and the neutral point is maintained viathe sliding contacts 4t) and the rails 38 and 39. During the initialpart of the movement the H.V. neutral point remains connected,substantially without any series resistance, to tapping A. With furthermovement of carriage 1, the discs 32 of cradle 10 meet the blocks 28 ofthe contact member 22 and make electrical contact therewith. The lengthof the copper end-block 24 of the contact member 23 is dimensioned sothat the leading roller assemblies of the cradle 11 roll on to the firstresistive block 25 of the contact member 23 as the leading rollerassemblies in the cradle 10 make contact with the blocks 28 of thecontact member 22. This means that the H.V. neutral point is connectedwith little or no resistance to tapping A and via the entire resistanceof the contact member 22, to tapping B. As the carriage 1 continues tomove away from the illustrated position towards the mid-point of itstravel, the total resistance in the circuit shorting the tappings A andB becomes less (since the roller assemblies 30 in cradle 10 are rollingover higher resistance blocks 25 on the contact member 22 than theroller assemblies in cradle 11 are rolling over on the contact member23) until it reaches a minimum value when the carriage is in itsmid-position. As the carriage now continues its movement, moreresistance is being added due to the carriage moving over the contactmember 23 than is being removed due to the carriage moving over thecontact member 22 so that the total resistance in the circuit shortingthe tappings A and B rises from the minimum value until it reaches avalue determined by the entire resistance of the contact member 22.Further movement of the carriage towards the right now results in theroller assemblies 30 in cradle 11 leaving the arc-resistant blocks 28 ofcontact member 23, so that tapping A is disconnected from the HIV.neutral point, leaving tapping B connected via little or substantiallyno resistance. The carriage 1 completes the movement as the blades 36become nipped in the jaws of the two pairs of contact clips 3712, thusconnecting the HV. neutral point solidly to tapping B. j

The tap change is complete and the tap selector can (if a furthertap-change in the same direction is desired), move the contact member 23from its connection with tapping A and place it into connection withtapping C ready for a fresh tap change on the return movement of thecarriage.

By arranging for the resistance in the shorting circuit of the tappingsto be reduced between the point where contact with the contact membersis made or broken and the mid-point of the switch travel, the resistivevoltage drop in series with the external circuit which occurs 1 during atap-change operation can be considerably reactual moment of make orbreak.

Many modifications can clearly be envisaged to the particular switchdescribed, for example the disposition and construction of the contactmembers could easily be changed and the sliding contacts 453 and rails38 and 39 could be completely dispensed with, the carriage 1 beingconnected directly to the requisite two sets of contact clips by abundle of flexible conducting braid. Sliding contacts may be employedbetween the carriage and the contact members in place of the rollerassemblies and alternative- -ly the sliding contacts 4d may be replacedby roller assemblies.

What is claimed is:

1. An electric load transfer switch, comprising housing means;

a movable contact positioned within said housing means and movablebetween spaced first and second positions;

moving means coupled to said movable contact for moving said movablecontact between first and second positions within said housing means andalong intermediate positions between said first and second positions;

a first stationary electrical Contact supported by said housing means ata position in said housing means in which said first stationary contactis engaged by said movable contact in said first position and for a potion of said intermediate positions adjacent said first position;

a second stationary electrical contact supported by said housing meansat a position in said housing means in which said second contact isengaged by said movable contact in said second position and for aportion of said intermediate positions adjacent said second position,said second stationary contact being spaced from said first stationarycontact, said first and second stationary contacts and said intermediatepositions and said movable cont-act being so positioned that saidmovable contact is disengaged from said first stationary contact and itsadjacent intermediate positions when engaged with said second stationarycontact and its adjacent intermediate positions and is disengaged fromsaid second stationary contact and its adjacent intermediate positionswhen engaged with said first stationary contact and its adjacentintermediate positions; p means supported in said housingmeanscontinuously i3 maintaining electrical contact with said movable contactthroughout the movement of said movable contact; and means cooperatingwith said movable contact and said first and second stationary contactsand said intermediate positions for maintaining electrical contactbetween said movable contact and each of said first and secondstationary contacts at said first and second position's'and formaintaining electrical contact between said movable contact and bothsaid first and second stationary contacts at a portion of saidintermediate positions, at least one of said first and second stationarycontacts having an electrical resistance varying in the directionextending between said first and second positions for at least a part ofthe portion thereof and of the adjacent intermediate positions engagedby saidmova-ble contact. 2. An electric load transfer switch as claimedin claim ll, further comprising a carriage slidably mounted in saidhousing means for movement between said first and second positions, saidmovable contact being supported by said carriage.

3. An electric load transfer switch as claimed in claim 1, wherein saidmeans continuously maintaining electrical contact with said movablecontact comprises an electrically conductive rail, electricallyconductive means on said movable contact and resilient means urging theelectrically conductive means on said movable contact into continuouselectrical contact with said electrically conductive rail. 4. Anelectric load transfer switch as claimed in claim 1, wherein said firstand second stationary electrical contacts are positioned in determinedrelation to each other by said housing means and have determineddimensions to provide electrical contact between said movable contactand both said first and second stationary contacts at a major portion ofsaid intermediate positions.

5. An electric load transfer switch as claimed in claim 1, wherein saidmeans for maintaining electrical contact between said movable contactand each of said first and second stationary contacts comprises trackmeans on said first and second stationary contacts and electricallyconductive roller means mounted on said movable contact and contactingthe track means of said first stationary contact and the track means ofsaid second stationary contact at determined times.

6. An electric load transfer switch as claimed in claim 5, wherein atleast a portion of the track means of each of said first and secondstationary contacts comprises arcresistant material.

7. An electric load transfer switch, comprising housing means;

an electrically conductive carriage positioned within said housing meansand movable between spaced first and second positions, said carriagehaving electrically conductive rollers mounted thereon;

moving means coupled to said carriage for moving said carriage betweenfirst and second positions within said housing means and alongintermediate positions between said first and second positions;

a first pair of substantially resilient contacts supported by saidhousing means at a position in said housing means in which said firstpair of resilient contacts is engaged by said carriage in said firstposition and at said intermediate positions;

a second pair of substantially resilient contacts supported by saidhousing means at a position in said housing means in which said secondpair of resilient contacts is'engaged by said carriage in said secondposition and at said intermediate positions, said second pair ofresilient contacts being spaced from said first pair of resilientcontacts;

means supported in said housing means continuously maintainingelectrical contact with said carriage throughout the movement of saidcarriage;

means cooperating with said carriage and said first and second pairs ofresilient contacts and said intermediate positions for maintainingelectrical contact between said carriage and one of the pair of contactsof each of said first and second pairs of resilient contacts at all ofsaid intermediate positions;

a first stationary electrical contact supported by said housing means,said first stationary electrical contact being electrically connected tothe other of the pair of contacts of said first pair of resilientcontacts and supported by said housing means at a position in saidhousing means in which said first stationary electrical contact iselectrically connected via selected ones of the rollers of said carriageto said carriage in said first position and for a portion of saidintermediate positions adjacent said first position; and

a second stationary electrical contact supported by said housing means,said second stationary electrical contact being electrically connectedto the other of the pair of contacts of said second pair of resilientcontacts and supported by said housing means at a position in saidhousing means in which said second stationary electrical contact iselectrically connected via selected others of the rollers of saidcarriage to said carriage in said second position and for a portion ofsaid intermediate positions adjacent said second position, said secondstationary contact being spaced from said first stationary contact, saidfirst and second stationary contacts and said intermediate positions andsaid carriage being so positioned that said carriage is disengaged fromsaid first stationary contact and its adjacent intermediate positionswhen engaged with said second stationary contact and its adjacentintermediate positions and is disengaged from said second stationarycontact and its adjacent intermediate positions when engaged with saidfirst stationary contact and its adjacent intermediate positions.

8. An electric load transfer switch as claimed in claim 7 wherein eachof said first and second stationary electrical contacts comprises aplurality of interconnected segments of different electrical resistance,the electrical resistance" 10 of said segments electrically connected tosaid carriage varying in magnitude at different ones of saidintermediate positions.

9. An electric load transfer switch as claimed in claim 7, wherein eachof said first and second stationary electrical contacts extends in adirection substantially toward the other for a distance greater thanhalf of the space between said first and second pairs of resilientcontacts so that at selected ones of said intermediate positions saidcarriage is electrically connected with both said first and secondstationary electrical contacts.

10. An electric load transfer switch as claimed in claim 9, wherein aportion of said first stationary electrical contact adjacent said firstpair of resilient contacts and a portion of said second stationaryelectrical contact adjacent said second pair of resilient contacts eachcomprise material of low electrical resistance, each of said first andsecond stationary electrical contacts having an electrical resistancewhich varies as each of said stationary contacts extends toward theother.

11. An electric load transfer switch as claimed in claim 10, wherein theelectrical resistance of each of said first and second stationaryelectrical contacts increases in magnitude as it extends toward theother of said stationary electrical contacts.

References Cited by the Examiner UNITED STATES PATENTS 1,433,740 10/22Nagel 338-97 X 1,867,147 7/32 Haller.

2,134,870 11/38 Fruth 338142 2,704,799 3/55 Kwasniewski 338126 3,030,5994/62 Bourns et a1 338-l83 OTHER REFERENCES German application,1,149,776, June 6, 1963.

KATHLEEN H. CLAFFY, Primary Examiner. BERNARD A. GILHEANY, Examiner.

1. AN ELECTRIC LOAD TRANSFER SWITCH, COMPRISING HOUSING MEANS; A MOVABLECONTACT POSITIONED WITHIN SAID HOUSING MEANS AND MOVABLE BETWEEN SPACEDFIRST AND SECOND POSITIONS; MOVING MEANS COUPLED TO SAID MOVABLE CONTACTFOR MOVING SAID MOVABLE CONTACT BETWEEN FIRST AND SECOND POSITIONSWITHIN SAID HOUSING MEANS AND ALONG INTERMEDIATE POSITIONS BETWEEN SAIDFIRST AND SECOND POSITIONS; A FIRST STATIONARY ELECTRICAL CONTACTSUPPORTED BY SAID HOUSING MEANS AT A POSITION IN SAID HOUSING MEANS INWHICH SAID FIRST STATIONARY CONTACT IS ENGAGED BY SAID MOVABLE CONTACTIN SAID FIRST POSITION AND FOR A PORTION OF SAID INTERMEDIATE POSITIONSADJACENT SAID FIRST POSITION; A SECOND STATIONARY ELECTRICAL CONTACTSUPPORTED BY SAID HOUSING MEANS AT A POSITION IN SAID HOUSING MEANS INWHICH SAID SECOND CONTACT IS ENGAGED BY SAID MOVABLE CONTACT IN SAIDSECOND POSITION AND FOR A PORTION OF SAID INTERMEDIATE POSITIONSADJACENT SAID SECOND POSITION, SAID SECOND STATIONARY CONTACT BEINGSPACED FROM SAID FIRST STATIONARY CONTACT, SAID FIRST AND SECONDSTATIONARY CONTACTS AND SAID INTERMEDIATE POSITIONS AND SAID MOVABLECONTACT BEING SO POSITIONED THAT SAID MOVABLE CONTACT IS DISENGAGED FROMSAID FIRST STATIONARY CONTACT IS DISENGAGED INTERMEDIATE POSITIONS WHENENGAGED WITH SAID SECOND STATIONARY CONTACT AND ITS ADJACENTINTERMEDIATE POSITIONS AND IS DISENGAGED FROM SAID SECOND STATIONARYCONTACT AND ITS ADJACENT INTERMEDIATE POSITIONS WHEN ENGAGED WITH SAIDFIRST STATIONARY CONTACT AND ITS ADJACENT INTERMEDIATE POSITIONS; MEANSSUPPORTED IN SAID HOUSING MEANS CONTINUOUSLY MAINTAINING ELECTRICALCONTACT WITH SAID MOVABLE CONTACT THROUGHOUT THE MOVEMENT OF SID MOVABLECONTACT; AND MEANS COOPERATING WITH SAID MOVABLE CONTACT AND SAID FIRSTAND SECOND STATIONARY CONTACTS AND SAID INTERMEDIATE POSITIONS FORMAINTAINING ELECTRICAL CONTACT BETWEEN SAID MOVABLE CONTACT AWND EACH OFSAID FIRST AND SECOND STATIONARY CONTACTS AT SAID FIRST AND SECONDPOSITIONS AND FOR MAINTAINING ELECTRICAL CONTACT BETWEEN SAID MOVABLECONTACT AND BOTH SAID FIRST AND SECOND STATIONARY CONTACTS AT A PORTIONOF SAID INTERMEDIATE POSITIONS, AT LEAST ONE OF SAID FIRST AND SECONDSTATIONARY CONTACTS HAVING AN ELECTRICAL RESISTANCE VARYING IN THEDIRECTION EXTENDING BETWEEN SAID FIRST AND SECOND POSITIONS FOR AT LEASTA PART OF THE PORTION THEREOF AND OF THE ADJACENT INTERMEDIATE POSITIONSENGAGED BY SAID MOVABLE CONTACT.